1. Field of the Invention
The present invention relates generally to magnetic disk drives (disk drives), and more particularly to an efficient method of manufacturing a disk drive by using a servo track writer (STW) for measuring the widths of the read and write elements to set the track pitch.
2. Description of the Related Art
Referring to FIG. 1, a conventional disk drive 10 has a head disk assembly (HDA) 20 including at least one disk 23, a spindle motor 22 for rapidly rotating the disk 23, and a head stack assembly (HSA) 40 that includes an actuator assembly 50 and a head gimbal assembly (HGA) (not numbered) with a transducer head 80 for reading and writing data. The HSA 40 is part of a servo control system that positions the transducer head 80 over a particular track on the disk to read or write information from that track. The HSA 40 earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical arrangement of heads called a “head stack.”
The transducer heads 80 of several years ago were “merged” devices where reading and writing were accomplished with a single inductive element. The transducer head 80 commonly used today, however, is a magneto-resistive transducer head 80 that has separate read and write elements. FIG. 2 is a highly simplified representation of a magneto-resistive transducer head 80 having it's a write element 81 of width W and it's a read element 82 of width R. The transducer head 80 shown is a “write wide, read narrow” device in that the read element's width R is typically about 50-65% of the write element's width W.
FIG. 3 is an exploded perspective view of a fully-assembled HDA 20 having servo-writing access ports 25, 26 (discussed below) and the controller circuit board 30 that is usually installed after servo-writing. The controller circuit board 30 suitably positions the actuator assembly 50 and then reads or writes user data in accordance with commands from a host system (not shown).
Returning to FIG. 1, the industry presently prefers a “rotary” or “swing-type” actuator assembly 50 that conventionally comprises an actuator body 51 which rotates on a pivot assembly between limited positions, a coil 52 that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm 54 that extends from the opposite side of the actuator body to support the HGA.
A disk drive is ultimately used to store user data in one or more “data tracks” that are most commonly arranged as a plurality of concentric data tracks on the surface of its disk or disks. Special servo information is factory-recorded on at least one disk surface so that the disk drive's servo control system may control the actuator assembly 50, via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the “your head is here” data it needs to attain and then maintain a desired head position. In operation, the disk drive's servo control system intermittently or continuously processes (read only) the pre-recorded servo information just before or while the disk drive processes (reads or writes) user data in the data tracks.
Earlier disk early drives used a “dedicated servo” system where one head and one disk surface provide the servo information for all of the other heads and disk surfaces. As shown in FIG. 4, however, the industry presently prefers an “embedded servo” system where the servo information is interspersed amongst the data on each surface of each disk. The factory-recorded servo information is contained in servo wedges 300 that are each divided into a plurality of servo sectors 310. The servo sectors 310 are recorded concentrically in order to provide numerous servo tracks (one entire rotation of servo sectors 310).
As shown, each servo wedge 300 generally comprises a header region (not separately identified) followed by a plurality of servo bursts. The header region fields include a setup or write splice field WRITE SPLICE, an address mark field AM, an automatic gain control/phase locked oscillator field AGC/PLO, a servo sync mark field SSM, a track identification field TKID, a wedge number field W#. The header region is followed by at least two servo bursts (an A burst and B burst are shown) that are circumferentially sequential and radially offset relative to a burst pair centerline. The servo format used is not critical and is explained here only for background purposes. The purpose of these various fields and available variations are well known to those of ordinary skill in the art.
The servo wedges 300 precede a corresponding number of data wedges 400 that are ultimately used to contain data tracks (not shown) that are divided into a plurality of data sectors (not shown). Each data wedge 400 may contain a whole or fractional part of one or more data sectors (not shown). Because the servo information is distributed around the disk within servo sectors 310, an embedded servo system is sometimes called a “sector servo” system.
The servo information is factory recorded at the time of manufacture using a relatively expensive and low-throughput manufacturing fixture called a servo track writer (STW). FIG. 5 is an exploded perspective view of a simplified servo-track writer (STW) 100 that is figuratively receiving an HDA 20 for servo-writing. The STW 100 records the servo information in special “servo tracks” on each surface of each disk for later use by the servo control system when the drive is in the hands of the user. The servo tracks are generally used throughout the life of the disk drive without modification. In recording the embedded servo information, the STW 100 take temporary control of the drive's write operation via a suitable electrical connector 102, repeatedly locates the write element 81 to a desired radial position, and then writes, erases, or does nothing (remains idle) at specific angular positions between the head and a reference position of the disk as the disk rotates beneath the write head. In order to precisely locate the write element 81 where needed, as shown in FIG. 3, a conventional HDA 20 has first and second access ports 25, 26 (later covered with adhesive labels) for allowing the STW to “reach in” and temporarily control the radial position of the actuator assembly 50 and measure the angular position of the disk while recording the servo information. As to the radial position of the actuator assembly 50, the conventional STW inserts a moveable “push pin” 101 into the first port 25, commands the HDA's VCM to bias the actuator assembly 50 against the push pin, moves the push pin 101 against the bias to move the actuator assembly 50 and the attached head 80, and measures the position of the push pin 101 with a laser interferometer to control the radial position of the head's write element 81 carried by the pin-guided actuator assembly 50. As to the angular position of the write element 81 relative to an index position of the disk, the conventional STW inserts a stationary “dock head” (not shown) into the second port 26, records a “clock track” containing thousands of “clock marks” and one “index mark” (e.g. an extra clock mark or a gap) on a topmost or bottom-most disk surface, and measures the angular position of the write element 81 relative to the index mark by detecting the index mark and thereafter tracking (i.e. counting) the intermediate clock marks.
The conventional STW embeds a servo pattern onto a disk by recording concentric servo tracks in a plurality of discrete “passes.” Each pass consists of moving the push-pin to “step” the transducer head to a desired radial position, allowing the head to “settle,” and during one ensuing revolution of the disk, writing new servo information, erasing overlapping portions of previously written servo information, or remaining idle (neither writing nor erasing). On the first pass, the STW moves the write head to an outer diameter of the disk, and then records magnetic transitions at discrete angular intervals to record the servo information including track identification (track ID) data and servo bursts. During the second and each of the thousands of subsequent passes, the STW steps the write head inward by a fraction of the intended data track pitch (e.g. ½ and ⅓ data track increments), waits for the write head to settle (as much as one full revolution), and then records the servo information during another full revolution, writing more magnetic transitions, trimming overlapping portions of previously recorded transitions, or holding idle, as appropriate for the desired servo pattern. In order to record each concentric servo track, therefore, the STW must repeatedly step, wait, and record.
The servo-writing process is a manufacturing bottleneck because each HDA must remain in the STW for an extensive amount of time in order to step, wait, and record each pass that collectively make up the required servo information.
Magneto-resistive transducer heads 80 are very small devices that are manufactured in large batches using photolithographic wafer process techniques. As a result, operating characteristics such as the widths of the read and write elements 81, tend to vary over a normal distribution curve for a given number of heads, wafers or manufacturers. The presence of separate read and write elements coupled with the wide variability of read width R and write width W is particularly troublesome as it relates to the servo-writing process and narrow range of widths that may presently be used.
In particular, the disk drive market is extremely competitive and drive makers are continually striving for manufacturing efficiencies, increased storage capacities, and higher performance in order to remain profitable. The servo-writing process is of major concern because STWs are so expensive (upward of $100,000) that only limited numbers can be used and it takes a long time to servowrite each disk drive (several minutes per drive). The servo-writing bottleneck is exacerbated by the fact that:                there is an ever increasing demand for areal density that can only be achieved with ever narrower data tracks (usually specified in tracks per inch or TPI) and ever tighter data densities (usually specified in bits per inch or BPI);        only a subset of the heads come from the manufacturer with read and write widths R, W that are suited for the nominal TPI. Some heads could be used with wider tracks, but they are not used at all. Some heads could be used with narrower tracks, but their enhanced capability is wasted with the nominal TPI, or worse, they are discarded altogether;        it is difficult to accurately identify this small subset of heads with currently available measurement techniques so, even if the heads are “binned” by making such measurements, many of the drives fail at a very late stage of the manufacturing process (after assembly of the HDA, servo-writing, mounting of the controller board and while testing the drive during Initial Burn-In or IBI), because one or more of the heads is too narrow or too wide. These failed drives must generally make a second trip through the servo-writing process, an unfortunate and expensive occurrence.        
Achieving efficiencies in terms of head use and the overall servo-writing process, therefore, may significantly reduce the overall cost of manufacturing disk drives. Consequently, there remains a need for a method of manufacturing a disk drive that allows more of the heads to be used in the first instance and that reduces the number of drives that must be re-worked and then take a second trip through the entire servo-writing process.